The present invention relates to industrial control systems and in particular to an I/O module for industrial control systems, the I/O module providing reduced power dissipation and a more compact form factor.
Industrial control systems were originally developed to provide control logic for equipment in factories operating to manufacture products on an assembly line or in batch processes. Early industrial control systems replaced racks of relays with specialized circuitry that mimicked these relays but used a programming language.
Current industrial control systems continued this evolution using modern electronic computer processors and circuits but may be distinguished from standard computers by architectures that address the needs for high reliability, safety, and resistance to environmental damage and normal contamination. This latter concern generally means that industrial control system components are designed without cooling fans or ports that communicate with the outside air, placing a premium on low heat dissipation.
Industrial control systems must function in a highly varied set of manufacturing applications. For this reason, industrial control systems normally employ a modular construction form in which one or more modular controllers are teamed with local or remote I/O modules and other control devices that provide the actual interface between the controller and the hardware of sensors and actuators driving the industrial process.
One type of input I/O module operates as an interface to receive an input signal from a sensor or the like, the sensor providing an output voltage in a first voltage range indicating a low state and an output voltage in a second voltage range indicating a high state. Generally, a low state may be represented by an input voltage between 0 and 10 volts and a high state may be represented by an input voltage between 10 to 32 volts.
It is important to prevent high electrical voltages originating in the controlled equipment or process from damaging the industrial control system. For this reason, I/O modules normally include an isolator. One possible isolator is an optical isolator (optoisolator/optocoupler) which provides an electrically actuated light source (an LED) insulated and electrically isolated from a photodetector (for example, a photodiode), the latter of which communicates with the remainder of the industrial control system. The optical isolator provides for good common mode rejection (that is, canceling noise that is present on both the ground and input side of the I/O module) and protects the industrial control system from extremely large voltage inputs which may accidentally be applied to the inputs of the I/O module.
Different devices attached to an I/O module may have different output voltage ranges with high state voltages ranging from as low as 5 volts to 30 volts or more. One way of accommodating this voltage range without exceeding the current capacity of the optical isolator is to use a series-limiting resistor. Such an approach may require different limiting resistors for different input voltage ranges and thus the fabrication of multiple I/O modules for different applications.
An alternative approach is disclosed in U.S. Pat. No. 6,043,703 to Bavol, assigned to the assignee of the present invention and hereby incorporated by reference. In this patent, a transistor-based, series current limiter is placed between one input of the I/O module and the light source of the optocoupler to limit the current to the optocoupler over the wide input voltage range. This active regulation of series current allows a single I/O module to work over a wide input voltage range.
It is important that the optical isolator not be activated by leakage current from the series current limiter. In order to ensure this result, it is normal to use optical isolators that are specially selected or specified to have relatively larger currents for activation.